This 'small grant' seeks to develop a method to apply neurotransmitters rapidly and focally in brain slice preparations in such a manner as to activate individual dendritic spines directly. Such a method could be used to distinguish presynaptic from postsynaptic actions in studies of synaptic plasticity, to map the distribution of glutamate receptors, and to study dendritic excitability. We have developed a method for light-induced release of glutamate from caged compounds under tissue culture conditions. This method relies on single-mode optical fibers to achieve saturating concentrations of glutamate within a 3 pm Spot in less than 1 ms. We now seek to apply this methodology to the brain slice preparation.